POLARISED LIGHT. 139 



the maximum effect, it is also requisite that the angle of 

 aperture of the polariser should be the same as the object- 

 glass, each ray of which should be directly opposed by a 

 ray of polarised light. The Polarising Condenser is merely 

 an ordinary achromatic condenser of large aperture, close 

 under the bottom lens of which is placed a plate of tour- 

 maline, used in combination with a superposed film of 

 selenite or not, as required . The effect of this arrangement 

 on some objects is very remarkable, bringing out strongly 

 colours which are almost invisible by the usual mode. 



The production of colour by polarised light has been 

 thus most clearly and comprehensively explained by Mr. 

 Woodward, in his " Introduction to the Study of Polarised 

 Light." 1 



, 



Fig. 87 A. 



abed represent the rectangular vibrations by which 

 a ray of common light is supposed tb be propagated. 



c, a plate of tourmaline, called in this situation the 

 polariser, and so turned that a b may vibrate in the plane 

 of its crystallographical axis. 



(1) Mr. Woodward constructed a very available form of polariscope for most 

 purposes ; the instrument is described in Element* of Natural Philosophy, by 

 Jabez Hogg. 



